How did I turn my victim into a phoenix-like hero?

I've been reborn as a background character in a game, and my childhood sweetheart is the protagonist's love interest—that's fine, but the problem is this is a pure hentai RPG with a NTR (Netorare) theme. The protagonist is guaranteed to become a NTR character on the first playthrough! You need enough resources for a pure love route on the second playthrough. As a player on my twentieth playthrough and a staunch advocate of pure love, I don't want to be a NTR character for even a second! To reach the pure love ending on my first playthrough, I must deceive the world.